Secure Shell（缩写为SSH），由IETF的网络工作小组（Network Working Group）所制定；SSH为一项创建在应用层和传输层基础上的安全协议，为计算机上的Shell（壳层）提供安全的传输和使用环境。

传统的网络服务进程，如rsh、FTP、POP和Telnet其本质上都是不安全的；因为它们在网络上用明文传送数据、用户帐号和用户口令，很容易受到中间人（man-in-the-middle）攻击方式的攻击。就是存在另一个人或者一台机器冒充真正的服务器接收用户传给服务器的数据，然后再冒充用户把数据传给真正的服务器。

Secure Shell, or SSH, is a cryptographic (encrypted) network protocol operating at layer 7 of the OSI Model to allow remote login and other network services to operate securely over an unsecured network.

SSH provides a secure channel over an unsecured network in a client-server architecture, connecting an SSH client application with an SSH server. Common applications include remote command-line login and remote command execution, but any network service can be secured with SSH. The protocol specification distinguishes between two major versions, referred to as SSH-1 and SSH-2.